1. Field of the Invention
The present invention relates to a signal processing circuit including an image sensor and, more particularly, to an improvement in a clamping operation which is performed by the processor.
2. Description of the Prior Art
An output signal of an image sensor is supplied through a pre-amplifier to a gamma correction circuit. Since the gamma correction circuit is the non-linear circuit, its input signal is needed to be matched with a constant DC level. On the other hand, the output signal of the image sensor has a variation in its DC potential, so that it is transmitted by being AC coupled to absorb this variation. For this purpose a clamping circuit is provided at the front stage of the gamma correction circuit.
The clamping circuit serves to clamp the image pickup signal so that the black level thereof becomes a predetermined DC potential. To this end a black level detecting portion (generally referred to as an optical black) consisting of a light shielding mask is provided in a portion of the image pickup surface of the image sensor (or image pickup device) and the interval corresponding to this black level detecting portion is clamped into a predetermined DC potential. However, since a noise is included in this interval of the black level detecting portion, the clamping circuit will have made the portion where the noise was added to the black level constant, so that it is impossible to expect the inherent operation to make the black level constant.
To reduce the influence of this noise, it is considered to perform the clamping operation with respect to the mean value of the black levels of a few times (in which the noise component was canceled) instead of the black level of one time. However, in this method, the influence of the noise can be fairly reduced, but on the contrary, there occurs a problem such that it is impossible to completely eliminate the sag component to be caused due to the AC coupling of the circuit at the front stage.